Socially and professionally, identification of an individual is an issue of growing importance. Passwords are commonly used, but can be shared and such sharing immediately defeats assurance of true personal identification.
Greater assurance of proper identification can be achieved with biometric verification. Voice print analysis, breath and retinal scanning have been recognized as effective means of identification, but each typically requires specialized equipment and backend processing. Fingerprint analysis has also proven to be an effective means of personal identification.
The fingerprint is comprised of a series of ridges and valleys of epidural tissue. It has been known for years that applying ink to a fingers permits an impression to be made that visually displays these ridges and valleys and permits individual identification. This impression is of course achieved because the ridges apply pressure to the paper and therefore transfer the applied ink, whereas the valleys apply no pressure and thus transfer little if any ink.
The presence of ridges and valleys in the finger print has permitted various systems to be devised that attempt to detect and recognize these same pressure point patterns in electronic form. Some of these include optical detection and others involve capacitance, and still others may employ physical pressure sensors. Indeed many computer systems now present the user with a sensor strip over which a user may drag his or her finger as a method of identification.
With respect to sensors employing physical pressure detecting devices, these are typically based on a cross point network. At each cross point there is a pressure activated contact and a resister in series. The matrix is implemented in a clam shell design. More specifically there are two substrates and each has a set of parallel features spaced about 50μ apart. On one substrate there is a stack of three thin films: a bottom layer of gold, a doped amorphous silicon layer which serves as the resister; and a second gold layer that makes up half of the mechanical contact. On this substrate all three layers are first patterned into lines and the top metal contact layer is then segmented into rectangles spaced at 50μ pitch to define individual elements at each crosspoint.
The silicon layer may be co-patterned with the top metal if its lateral conduction is significant relative to its conduction through the film. The second substrate has a set of parallel metal lines on it. The substrates are assembled with the patterned films facing each other. The parallel features on each substrate must be properly aligned so that they are orthogonal with respect to those on the other substrate.
The substrates are assembled so that the metal contacts on top of the silicon films are aligned with the metal traces on the opposing film. The substrates are held apart by some form of spacer that must also be aligned and positioned so as not to prevent contact between the metal contacts on top of the silicon films and the metal traces. A typical fingertip sensor contains 256 rows and 256 columns resulting in a sensor of approximately 13×13 mm.
When a finger is pressed down on the sensor, the substrates are forced together under the ridges closing the electrical contacts at those locations in the cross point array. Between the ridges no electrical contact is made. The pitch of the ridges on the fingertip is approximately 400μ.
The detection of a single open or closed switch in such a crosspoint resister array is a difficult problem. If all the switches are closed and voltage is applied to one row and one column, then in addition to the current that is flowing through the switch at the selected row and column, there will be additional currents flowing through all the unselected row and column resisters to ground. The ‘sneak path current’ is equal to 510 1MΩ resistors (the non-selected resistors on the selected row and column) in parallel through half the voltage drop or 255 resistors in parallel through the whole voltage drop.
This problem is not substantially changed if the unselected lines are left floating instead of being grounded. The detection problem is equivalent to detecting the difference between 255 and 256 resistors in parallel. Because these resistors ate vertical thin film resistors their resistance is hard to control and the variability will further complicate the detection problem. This detection can be made easier if the common mode signal (sneak path current) can be turned off, however this is difficult because it is data (fingerprint) dependent and will take time to isolate. Readout is also slow as it is made one pixel at a time.
Moreover, large crosstalk in the device makes detection difficult. In addition, the use of gold is expensive and the issue of precise alignment makes fabrication difficult. In the event of a short, the short will kill the row and column that it is located on. Multiple shorts will quickly result in a non-viable crosspoint array.
Due to the these technical issues, it is often more common to see applications of a sensor strip across which the fingertip is swiped. As the swipe sensor is narrower in at least one dimension it is lower in cost and it lessens some of the above issues, though they still remain.
With respect to the fabrication of the substrates, each is fabricated through traditional photolithography. In a photolithographic process, a substrate is provided and at least one material layer is uniformly deposited upon the substrate. A photo-resist layer, also commonly known simply as a photoresist, or even resist, is deposited upon the material layer, typically by a spin coating machine. A mask is then placed over the photo resist and light, typically ultra-violet (UV) light, is applied. During the process of exposure, the photoresist undergoes a chemical reaction. Generally the photoresist will react in one of two ways.
With a positive photoresist, UV light changes the chemical structure of the photoresist so that it is soluble in a developer. What “shows” therefore goes, and the mask provides an exact copy of the patterns which are to remain—such as, for example, the trace lines of a circuit.
A negative photoresist behaves in the opposite manner—the UV exposure causes it to polymerize and not dissolve in the presence of a developer. As such the mask is a photographic negative of the pattern to be left. Following the developing with either a negative or positive photoresist, blocks of photoresist remain. These blocks may be used to protect portions of the original material layer, serve as isolators or other components.
Very commonly, these blocks serve as templates during an etching process, wherein the exposed portions of the material layer are removed, such as, for example, to establish a plurality of conductive rows.
The process may be repeated several times to provide the desired thin film devices. As such, new material layers are set down on layers that have undergone processing. Such processing may inadvertently leave surface defects in the prior layers as well as unintended contaminant particles.
The crystalline texture of the materials composing each material layer, and specifically the crystalline texture of each material at an interface between materials is often of significant importance to the operation of the thin film device. Surface defects and surface contaminants may negatively affect the interfaces between layers and possibly degrade the performance of the thin film device.
In addition, photolithography is a precise process applied to small substrates. In part this is due to the high cost of the photo masks. For the fabrication of larger devices, typically rather than employing a larger and even more costly photo mask, a smaller mask is repeatedly used—a process that requires precise alignment.
As a photolithographic process typically involves multiple applications of materials, repeated masking and etching, issues of alignment between the thin film layers is of high importance. A photolithographic process is not well suited for formation of thin film devices on flexible substrates, where expansion, contraction or compression of the substrate may result in significant misalignment between material layers, thereby leading to inoperable thin film devices. In addition a flexible substrate is not flat—it is difficult to hold flat during the imprinting and/or exposure process, and thickness and surface roughness typically can not be controlled as well as they can for glass or other non-flexible substrates.
The issue of flatness in photolithography can be a problem because the minimum feature size that can be produced by a given imaging system is proportional to the wavelength of the illumination divided by the numerical aperture of the imaging system. However the depth of field of the imaging system is proportional to the wavelength of the illumination divided by the square of the numerical aperture. Therefore, as resolution is increased the flatness of the substrate quickly becomes the critical issue.
These issues of fabrication are of concern in the fabrication of a fingerprint sensor and typically limit the size of such a device to the 256 by 256 pixel device noted above. As applications may well exist where a larger device would be well suited, multiple small devices must be combined, each of which enjoys the same problems and limitations in performance and fabrication as noted above.
Hence, there is a need for a process to provide at least one pressure sensor thin film device that overcomes one or more of the drawbacks identified above.